Weed control within organic blocks is more critical than in conventional blocks. Weedy orchards can compete for the high value organically certified nutrients, decreasing yields and/or increasing fertilizer input costs. Organic weed control methods are also not as effective and more costly than conventionally available materials. Therefore it is important for growers to use multiple strategies to reduce weeds to a level that is economically acceptable. This entry will focus on several methods available for organic almond production weed control. These methods may also be useful in conventional orchards that have multiple areas of herbicide resistant weeds.
Mulches: Plastic or fabric mulches block light, preventing weed germination or growth. These materials are often placed in the row strip before planting. Upon planting, a hole is made at the planting location so that the tree can be planted. Placing the mulches post planting can also be done. These materials control most weeds effectively while they are still intact. Materials do degrade over time due to UV light and temperature. Weed seeds that land on top of the mulches can germinate and grow. These types of materials are expensive ($250-$300 treated acre) and post-use disposal can be problematic since they are currently are not able to be recycled. Organic mulches (straw, newspaper, wood chips) can be used for early season weed management, but often interfere with harvesting practices. Organic mulches are most effective when they are at least four inches thick. Use on non-bearing age trees may be feasible. Remember, these mulches MUST be organically certified to be used.
Cultivation: This is the most widely used organic weed control method. Cultivation uproots and buries weeds. This tends to work better on smaller, shallow rooted weeds. Deep cultivation is not advisable since it can damage tree roots. Usually, a mix of a tractor mounted cultivator with some hand weeding is used. Semi-recent research suggests that tillage during the night may increase the effectiveness. Growers need to be careful about irrigation lines when using this practice.
Mowing: This falls under a similar category as cultivation. Many growers mow in between the rows and trees, doing a relatively thorough job in weed management. Before harvest they often spend a little extra time to hand weed missed weed patches to ensure a thorough/efficient harvest. Irrigation lines often interfere with cross-mowing. Also, care should be taken so that the tractor operator does not damage the tree.
Organically Approved Herbicides: Varying in effectiveness, these herbicides usually contain naturally found plant based oils. Applied at high concentration, these oils will kill anything that is green due to their ability to damage the plant’s cuticle and epidermal cells. Good coverage is essential and most appear to be safe if accidentally sprayed on the trunk. Weed size is also critical when using herbicides. Studies have shown that most available products provide adequate control of weeds up to 2 inches in height. Results were less satisfactory on larger weeds. Organic herbicides tend to be expensive and will need multiple applications – just like any other method. Effectiveness may be increased by the addition of surfactants and efficient application methods.
Water Management: Buried drip tape can place water within the tree’s root zone, but yet out of reach of weed roots. As the season progresses, the climate of the central valley will dry out and kill most weeds, eliminating the need for other control methods. This method is highly effective in reducing unwanted vegetation and helps increase water use efficiency. However, the use of buried drip tape does have some drawbacks. These include plugged emitters, vertebrate pest damage, and lack of water movement within the soil profile. It is important to note that other irrigation systems that reduce the available water to weeds will also provide some level of control.
Flame/Steam Weeding: Propane fueled burners provide an effective way for weed control. Brief intense heat (130 degrees F) ruptures cell walls by causing the cell sap to expand. Weeds must have less than two true leaves for the greatest efficiency. Grasses tend to be more difficult to control since the growing point is underground. Efficiency can be improved by burner placement, heat source, flaming angle, flaming pattern, and flaming length. These vary by manufacturer. Efficiency is increased by flaming in calm conditions. Care must be used to prevent tree damage and reduce the risk of fires. Generally, this method tends to be very effective, but the required equipment can be costly and may be subject to air quality mandates. Steam can also be used to generate a similar affect as the burners, but this is newer technology, (currently) tends to be less effective than flaming, and can be more expensive.
Please let me know if you have any questions/comments/concerns regarding the outlined methods – or if I missed a potential method!
Information used was based off of W. Thomas Lanini’s 2009 Weed Short course article, “Organic Weed Management.”